What is it for?

[tromic]

I find this front shape of the piston on different pistons.



Is it for noise reduction or it is to center the piston? Peter, maybe you know the answer?

 

[popgun pete]

I do not know what those small bumps on the face of the piston are for, if they hit the shock absorber anvil face directly then they would tend to be flattened over time, however they may just sit inside the bore hole through the anvil (or stopper) and thus centre the piston. On metal pistons the curve on the outer periphery of the piston nose closely matches the curvature of the concave anvil face profile and centralizes the piston when it is resting against the anvil. Over time and with lots of shots the piston hammers the anvil into shape when both these items are metal, so they match up even better, it can even cause the piston to stick in the anvil, but it is easily freed if you push hard enough with the loading bar. That hammering to shape is unlikely to occur with plastic, in fact the parts may crack with repeated impacts at high pressure operation which the manufacturers discourage users from trying in modern guns.

It is possible that the bumps are an artifact of how the piston is made in terms of being ejected from the mould, could be ejector pins are just below flush in the injection moulding die when the plastic flows in and that creates a ring of bumps on the face of the piston.

Check the shoulder step of the anvil recess where the piston fits into it, if there is no matching curve there to the piston nose shape and the shoulder is squared off instead then the bumps may be to centralize the piston on the bore hole rather than at the shoulder of the anvil.

 

[tromic]

I do not know what those small bumps on the face of the piston are for, if they hit the shock absorber anvil face directly then they would tend to be flattened over time, however they may just sit inside the bore hole through the anvil (or stopper) and thus centre the piston. On metal pistons the curve on the outer periphery of the piston nose closely matches the curvature of the concave anvil face profile and centralizes the piston when it is resting against the anvil. Over time and with lots of shots the piston hammers the anvil into shape when both these items are metal, so they match up even better, it can even cause the piston to stick in the anvil, but it is easily freed if you push hard enough with the loading bar. That hammering to shape is unlikely to occur with plastic, in fact the parts may crack with repeated impacts at high pressure operation which the manufacturers discourage users from trying in modern guns.

It is possible that the bumps are an artifact of how the piston is made in terms of being ejected from the mould, could be ejector pins are just below flush in the injection moulding die when the plastic flows in and that creates a ring of bumps on the face of the piston.

Check the shoulder step of the anvil recess where the piston fits into it, if there is no matching curve there to the piston nose shape and the shoulder is squared off instead then the bumps may be to centralize the piston on the bore hole rather than at the shoulder of the anvil.

Thanks, Peter. There is matching curve to the piston nose shape.

Cheers, tromic

 

[popgun pete]

I checked the plastic pistons of a "Cyrano" and a "Sten", I can easily see the eight bumps on the visible face of the piston looking down the muzzle (with the help of a small torch), so the bumps do not strike the anvil at all. On the "Sten" the outer edges of the round bumps are right near the edge of the bore through the anvil (or piston shock absorber stopper) and at their inner edges they sit on the border of the chamfer that leads into the spear tail receiver. On the "Cyrano" there is no appreciable chamfer, so the inner edges of the bumps sit right on the edge of the spear tail receiver and at their outer edges they have a small clearance to the bore through the anvil, at least there is a tiny gap there not evident on the "Sten". The "Sten" is a "Competition Line" model which pre-dates the "Cyrano", so those bumps have been a feature on the pistons for some time.

 

[tromic]

Maybe those bumps are to easy water drainage from the muzzle when the gun is muzzle down?

 

[popgun pete]

I do not think so as the bumps appear to not contact anything, they do not add to any clearance, in fact they are very small on the "Cyrano" piston compared to the "Sten", chiefly because the "Sten" piston is 13 mm rather than 11 mm diameter. The other reason bumps are sometimes added to the face of a cylindrical item is to act as a key when turning the object with a special tool that has corresponding hollows to match the bumps and engage them thus allowing you to twist the object from the front end. I think that they are too insubstantial for this task, but maybe they are a manufacturing aid in the production/assembly process and have no relevance to service use.

 

[tromic]

There are no bumps on Cressi, Seac guns. Only Mares.